A wide variety of manufacturing operations are performed in a variety of industrial environments on sheet material in the form of a moving web in which the web is advanced past various operating stations, each of which performs a particular operation on the web. For example, in the manufacture of some types of packaging bags, as from thin plastic sheeting or the like, it is desirable to punch wicket holes at predetermined spaced locations along the web to facilitate holding the bag while it is being filled with the product to be packaged. In this particular manufacturing operation, the holes must be punched in precisely the correct location so that the bag may be strong enough to hang on the pins or wickets while the bag is being filled. The holes also must be located to enable the bag to be torn from the supporting wickets after the bag has been filled. Proper formation and location of the holes is essential particularly when the bag filling procedure is automated. For example, if the holes are not at precisely the right location and the bag tears prematurely or fails to tear at the proper time, a fully automated packaging system may have to be shut down until suitable repairs and/or adjustments are made.
In general, proper location of such hole punching or similar devices has been effected by a technique employing a proximity sensor to sense the location of the edge of the moving sheet. The sensor device is operatively associated with a control arrangement for continually guiding and repositioning the web toward and away from the operating station. For example, U.S. Pat. Nos. 3,759,457 and 3,785,542 show such prior devices in which the moving web is positioned by shifting rollers and other web guiding devices to move the web toward or away from the operating station. Such techniques are both complex and costly and are not free from difficulty. Moreover, they are generally unsuited for use in environments where the web moves at a relatively high speed because their response times, from the time the location of the edge of the web is sensed to the time the web shifting mechanism is operated, often is too great.
The present invention is intended to overcome the difficulties inherent in such prior systems and in a manner which is relatively simple and inexpensive.